The technique of providing barrier seals in soil in the form of cast walls is very frequently employed in hydraulic structures, e.g., for stopping the flow of subterranean waters with a possibility of creating an upstream reservoir or forming a dried region out of water downstream, sealing of flow channel dikes, separation of water of different salinities, and the like.
Sealing barriers or walls cast in the soil are produced by making a trench with the aid of a tool which excavates the ground and by replacing the excavated ground with a material intended to ensure the sealing. When the ground exhibits a low cohesion and when it is a question of producing a sealing wall of moderate depth, they can also be produced by making a series of impressions with the aid of a tool which is forced into the soil and by filling these impressions with a material of the grout or mortar type ensuring the sealing of the barrier thus created at the time when the tool is withdrawn.
Barrier seals are also employed in the field of environment protection so that polluted waters contaminated by harmful effluents are prevented from contaminating conserved regions. Harmful effluents are chiefly produced by industrial activities which accidentally or otherwise discharge chemical effluents into phreatic tables or into surface waters. When a barrier seal intended to conserve regions not affected by the contamination produced by polluted effluents is constructed, a problem arises concerning the longevity of the constituent material of the barrier facing the variety of chemical products liable to be present in the effluents.
In conventional works forming part of hydraulic structures, the leakproof materials making up the walls are very generally made up of grouts, mortars or sealing concretes. The grouts are prepared based on clay, more particularly bentonite, a hydraulic binder which is generally a cement and, optionally, a filler which may be inert or optionally reactive. Mortars and concretes comprise the same types of materials as grouts but, in addition, they contain a large quantity of granulates which will make up a granular skeleton. In mortars, the granulates are limited to sandy materials whereas in concretes the granulates include gravels in addition to the sandy materials.
When these materials are intended for forming screens intended to retain waters of an acidic nature, the presence of hydraulic cement (be it of Portland-type cements or of slag-based cements, which are the types most commonly employed) constitutes a point of uncertainty with regard to the longevity of the wall sealing.
If the acidity of the water liable to percolate through the sealing wall is characterized by its pH value, it is accepted that in the case of pH values from 7 to approximately 5.5 the waters are considered as slightly corrosive towards cements, whereas in the case of lower values the waters progress from a strongly corrosive to a very strongly corrosive nature as their pH falls.
Such waters are sometimes encountered in the subsoil, and the construction of walls whose sealing endures in a medium with a pH below 5.5 does not permit the incorporation of a conventional hydraulic cement, which would be destroyed in time by the acidity of the percolating waters, thus damaging the intended mechanical characteristics and sealing characteristics.
A method for reducing the risk of damage of the cement binder by the acidic waters consists in eliminating the problem by doing away with the use of cement. In this case, the grouts which are usually made up of water, clay and/or bentonite and cement can no longer be envisaged because, without cement, these grouts are merely a slurry without cohesion and without stability. The materials for filling barrier seals are then made up of mortars or of sealing concrete which themselves comprise a granular skeleton ensuring the longevity of the volume of material. The sealing is provided by the clay, and more particularly by bentonite, which fills the interstices included between the particles of the granular skeleton.
The granular skeleton exhibits particle size distribution characteristics such as to make it self-filtering and that, in particular, the clay material runs no risk of being entrained by the percolation stream. This characteristic is obtained by the presence, arising naturally or by addition, of an appropriate quantity of fine materials with a particle size of between 0.002 mm and 0.1 mm.
This method is of a certain interest for preventing short-term destruction of the sealing against the percolation of corrosive acidic waters, but is unsatisfactory in the long term, because permeability to corrosive acidic waters increases with time. In any event, it is unsuitable when the acidic waters are highly corrosive.
This is demonstrated by an accelerated laboratory test:
a sealing concrete of the following composition by weight:
______________________________________ siliceous gravel (5-8 mm) = 311 kg siliceous sand (&lt;5 mm) = 1,240 kg siliceous fines (0.002-0.1 mm) = 449 kg bentonite = 40 kg water = 386 kg ______________________________________
is placed in a permeability test cell and is subjected to a percolation gradient (ratio of the water pressure, expressed in cm of water, to the thickness, expressed in cm, of the sealing concrete layer tested) of 330 with a water whose pH is adjusted to the value 0 by adding sulfuric acid. The initial value of the permeability coefficient, which is 2.times.10.sup.-9 m/s remains stable for 6 minutes, and then rapidly increases to reach the value of 10.sup.-8 m/s at the end of 15 minutes of percolation test, and continues to increase thereafter.
There is therefore an existing need for a process which makes it possible to produce sealing walls which durably withstand strongly corrosive, to very strongly corrosive, acidic waters.
The invention aims to meet this need, to which no satisfactory solution exists at this time.